1. Field of the Invention
This invention relates generally to methods for crimping expandable medical devices, more particularly, to methods and processes for crimping medical devices constructed of superelastic alloys.
2. Description of the State of the Art
Materials, organic and metallic, are capable of possessing shape memory. A device made of such materials can be deformed from an original, heat stable configuration to a second, heat-unstable configuration. The device is said to have shape memory for the reason that, upon the application of heat alone, it reverts, or to attempts to revert, from its heat-unstable configuration to its original, heat stable configuration. That is, the device “remembers” its original configuration with the application of heat.
Among metallic alloys, the ability to possess shape memory is a result of the alloy undergoing a reversible transformation from an austenite molecular structure to a martensite molecular structure with a change in temperature. An alloy having an austenite or martensite molecular structure are generally described as austenitic or martensitic, respectively. Alloys capable of this transformation are known generally as shape memory alloys (SMAs). This transformation is sometimes referred to as a thermoeleastic martensitic transformation. An article made from an SMA, a hollow sleeve, for example, is easily deformed from its original configuration to a new configuration when cooled below the temperature at which the alloy is transformed from the austenitic state to the martensitic state. The temperature at which this transformation to the martensitic state starts is referred to as Ms and the temperature at which it finishes is referred to as Mf. That is, the transformation to the martensitic state occurs over a temperature range of Ms to Mf. When the article is deformed and later warmed to the temperature at which the alloy starts to revert to the austenitic state, referred to as As, austenite structures begin to form in the alloy and are interdispersed among martensite structures. At about this stage, the deformed article will begin to return to its original, undeformed configuration. The temperature at which the alloy reverts completely to the austenite state is referred to as Af.
Many shape memory alloys (SMAs) are known to exhibit stress-induced martensite (SIM). When an SMA sample exhibiting stress-induced martensite is stressed at a temperature above Ms (so that the austenitic state is initially stable), but below Af (the maximum temperature at which martensite formation can occur even under stress) it first deforms elastically and then, at a critical stress, begins to transform by the formation of stress-induced martensite. The behavior of the SMA sample when the deforming stress is released differs depending on whether the temperature of the SMA sample is above or below As. If the temperature is below As, the stress-induced martensite is stable; but if the temperature is above As, the martensite is unstable and transforms back to austenite, with the SMA sample returning (or attempting to return) to its original shape. This temperature-dependant behavior when stress is released, along with the shape memory effect, is seen in almost all alloys which exhibit a thermoelastic martensitic transformation. However, the extent of the temperature range over which the SIM is seen and the stress and strain ranges associated with the above-described temperature-dependant behavior vary greatly among SMAs.
SMAs are commonly utilized in the medical field. For example, U.S. Pat. No. 3,620,212 to Fannon et al. proposes the use of an SMA intrauterine contraceptive device; U.S. Pat. No. 3,786,806 to Johnson et al. proposes the use of an SMA bone implant; and U.S. Pat. No. 3,890,977 to Wilson proposes the use of an SMA element to bend a catheter or cannula.
A more common application for SMAs are medical devices such as stents, which can be relatively small and intricate. For example, U.S. Pat. No. 7,128,756, entitled “Endoprosthesis Having Foot Extensions,” the entirety of which is hereby incorporated by reference, illustrates exemplary embodiments of stents which may be formed of an SMA such as Nitinol.
These medical SMA devices rely on the property of shape memory to achieve their desired clinical effect. That is to say, they rely on the characteristic, that when an SMA element of the device is cooled while in its original shape and size to its martensitic state and is subsequently deformed, it will retain its deformed shape and size; and when it is warmed to its austenitic state, the original shape and size will be recovered.
Stents formed of an SMA are generally heat set to an expanded diameter. Thus, to facilitate delivery within a vessel or artery, they must be crimped to and retained at a smaller diameter. The stents are usually crimped utilizing a stent crimping apparatus such as shown and described in U.S. Pat. No. 6,629,350. The crimped stent is then placed into a delivery device. U.S. patent application Ser. No. 10/932,964, entitled “Delivery System for a Medical Device,” the entirety of which is herein incorporated by reference, shows an exemplary stent delivery system, wherein a stent is disposed within a receiving area at the distal end of the delivery system and a slidable sheath covers the stent. To deliver the stent, the delivery system is tracked over a guidewire to a desired location wherein the slideable sheath is retracted from the covering the stent, thereby allowing the stent to expand from its crimped diameter to its expanded diameter.
However, the use of the shape memory effect in the field of stents has a disadvantage in that it is difficult to crimp the stent at or near room temperature and load it into a delivery device without damaging the stent or the delivery device. Additionally, at or near room temperature, the stent is very bendable and flexible, and may have high hoop strength.
Therefore, there is a need for a process for crimping and loading stents made of SMA that allows for easy loading and reduces the risk of damaging the stents or the delivery system during the crimping/loading process. There is also a need for a method of crimping a stent constructed of an SMA that takes advantage of its shape memory property, i.e., its ability to return to an original shape after relatively substantial deformation, without hindering the crimping or loading process. The present invention satisfies these and other needs.